Solar system debate about Pluto provokes a space squabble

Leading astronomers are bitterly divided over new galactic guidelines that for the first time would define what is and isn't a planet.

The debate all but dooms a proposal being put to a vote today to expand the solar system to 12 planets from the traditional nine.

Caught yet again in the crossfire is puny Pluto, scorned by many as a poser that could be demoted as a dwarf - slightly shrinking Earth's neighborhood instead.

Opponents "smell blood, and I think they're going to get it," Alan Boss, an astrophysicist at the Carnegie Institution in Washington, D.C., said on the eve of a vote by members of the International Astronomical Union.

Leaders of the group, the official arbiter of heavenly bodies, caused a sensation last week by proposing that Pluto's largest moon and two other objects officially be designated as planets. They suggested that Pluto and the three newcomers be the first of a new class of planet dubbed "plutons."

The rationale was their initial draft definition of a planet: any object larger than nearly 500 miles in diameter that orbits the sun, has a mass roughly one-12,000th that of Earth and has enough self-gravity to pull itself into a round shape.

But for many of the 2,500 astronomers from 75 countries meeting in Prague, the universe hasn't been the same since.

After days of spirited and sometimes combative debate, renegade scientists have won some key concessions.

A planet, they insist, must be the dominant object in its area. That would draw a sharp distinction between the eight "classical planets" - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune - and Pluto, which would be known as a "dwarf planet."

Defining the solar system

The precise wording of the definition remained a work in progress Wednesday. However, if astronomers agree that a planet must have "orbital dominance" in its own neighborhood, the new guidelines would eliminate Pluto and the trio of tentative candidates as proper planets.